Many systems include imaging devices to sense and capture optical images that can be electronically converted to a digital representation of the image. These image sensors include an array of photo-sensitive devices such as photodiodes, photo-transistors, photoconductors, or photogates, fabricated on, for example, a complementary metal oxide semiconductor (CMOS) device. The photo-sensitive devices are arranged as an array of pixel cells in a focal plane. Each photo-sensitive device is sensitive to light in such a way that it can create an electrical charge that is proportional to the intensity of light striking the photo-sensitive device. The overall image captured by an image sensor includes many pixels arranged in an array such that each pixel detects the light intensity at the location of that pixel. A single pixel may include a single photo-sensitive device configured for detecting a broad frequency range, which may be used for gray scale images. In addition, a pixel may be defined as a single photo-sensitive device configured for detecting a specific color (i.e., frequency). Finally, a pixel may be defined as a group of photo-sensitive devices arranged near each other wherein different devices within the group are configured for detecting different colors. Thus, a full color image may be detected with the proper combination of color sensing pixels.
Many conventional CMOS imagers include isolation regions around devices in the pixel array and isolation regions around devices in a periphery region outside of the pixel array. These isolation regions may assist in preventing leakage current between the various devices. Particularly with imagers, it is important to reduce leakage currents that may adversely affect the charge collected on photodiodes. This leakage current to photodiodes may manifest as dark-current or as a coupling between adjacent pixels.
Isolation regions may be formed in different ways. For example, shallow trench isolation (STI) regions have been used, doped regions have been used, and a Local Oxidation of Silicon (LOCOS) process has been used. Each of these isolation types may have benefits and disadvantages. For example, a common problem associated with the formation of STI regions is that, when ions are implanted in the substrate close to the bottom and edges or sidewalls of the trench, current leakage can occur at the junction between the active device regions and the trench resulting in “hot pixels,” (i.e., pixels that may appear brighter than other pixels). Minimizing hot pixels and dark current in a photodiode is important in CMOS image sensor fabrication. However, isolation between pixels and various devices in the pixel array is also important. As a result, in many cases, isolation regions are a trade off between hot pixel effects and coupling between adjacent devices. Furthermore, a type of isolation that is advantageous for pixel array devices may not be as advantageous for devices in the periphery logic.
There is a need for image sensors with isolation regions in the pixel array that differ from isolation regions in the periphery around the pixel array.